The present invention is related to methods and apparatus for detecting terminal open circuits and short circuits to ground in inductive head write driver circuits. More particularly, the present invention is related to methods and apparatus for detecting terminal open circuits and short circuits to ground in inductive heads using voltage-mode write drivers.
Today""s disk drives store data on magnetic rotating disks, or platters. These platters are typically made from glass or ceramic, and have a layer of magnetic material deposited on their surface. Data is stored in the form of binary digits transmitted to the disk drive in a corresponding time sequence of binary xe2x80x9conexe2x80x9d and xe2x80x9czeroxe2x80x9ddigits, or bits. These bits are converted into an electric current waveform that is delivered by wires to a write head. The write head is used to store the digital information on the magnetic platters.
A typical write head comprises a spiral inductive coil wrapped between two layers of soft magnetic material. At one end of the head, there exists a gap between the two magnetic layers, while at the other end of the head the magnetic layers are joined together. The layers of magnetic material are readily magnetized when an electric current flows in the inductive coil. This results in the layers becoming opposite magnetic poles of an extremely small electromagnet.
Data is typically stored on the platters by sending pulses of current from the drive electronics of the disk drive to the head assembly. The direction of the current, and thus the direction of the diverging magnetic field across the gap in the head, determines the magnetic polarization at any given location on the platter""s magnetic coating. A timing clock may be synchronized to the turning of the platters, such that bit cells are formed corresponding to portions of the platter surface area. Some of these bit cells may be used store digital xe2x80x9conesxe2x80x9d, while others may be used to store xe2x80x9czeroesxe2x80x9d. Once written, the bits stored at the disk surface are xe2x80x9cpermanentlyxe2x80x9d magnetized in either one direction or the other, until new data patterns are written over the previously stored patterns.
In a simple data storage scheme, a xe2x80x9conexe2x80x9d bit may be encoded to correspond to a change in current polarity, while a xe2x80x9czeroxe2x80x9d bit may be encoded correspond to no change in polarity of the writing current. A moving disk may thus be magnetized in a positive direction for positive flowing current, and be magnetized in a negative direction for negative flowing current. This simple scheme results in the stored xe2x80x9conesxe2x80x9d being represented by reversals in magnetic polarization on the surface of the disk, with the stored xe2x80x9czeroesxe2x80x9d residing on the disk between the xe2x80x9conexe2x80x9d values.
Two types of head malfunctions can cause information to not be written to the disk properly. One type of head malfunction is an open head condition. An open head condition occurs when the inductive coil breaks or does not make electrical contact with the head contacts. In other words, the inductive coil creates an open-circuit between the head contacts. The other type of head malfunction is a head short circuit condition. A head short circuit condition typically occurs when one of the head contacts becomes shorted to ground.
Disk drives typically include open and short-circuit head detection circuitry that can perform fault detection on the inductive head. During operation, if the inductive head malfunctions, this circuitry notifies the storage system of the malfunction by way of fault signals to prevent the disk drive system from attempting to write data through the defective head. An example of an open-circuit detection arrangement is described in U.S. Pat. No. 5,592,097 to Shimizu et al., entitled xe2x80x9cLoad Open State Detection Using H-Bridge Driving Circuit.xe2x80x9d An exemplary short-circuit detection arrangement is described in U.S. Pat. No. 5,434,717 to Yoshinaga et al., entitled xe2x80x9cRead and/or Write Integrated Circuit Having an Operation Timing Adjusting Circuit and Constant Current Elements.xe2x80x9d Typically, this fault detection circuitry is included in the drive electronics of the disk drive.
The drive electronics include write driver circuitry (or write drivers) that deliver the needed write current to the inductive head to polarize the platter bit cells. Conventional write drivers operate as current-mode devices that deliver the write current to the head using programmable current mirrors. Conventional current-mode writer drivers, such as the write driver 100 (circuitry to the left of the dashed line) shown in FIG. 1, typically use an on-chip damping resistor 102 in parallel with the head assembly 104 to lower the output impedance of the write driver. The damping resistance 102 must be considerably larger than the impedance of the head assembly 104 to avoid shunting excessive DC write current IWC. Another purpose of the damping resistor 102 is to provide a current path for IWC in the event that one of the head terminals 106 become open-circuited. When an head terminal open-circuit condition occurs, the increased voltage across the damping resistor 102 may be detected to notify the disk drive system of the open head fault.
Two methods may be used to detect a short-circuit to ground condition in the head terminals 106 of the current-mode write driver 100. In the first method, the average voltage across the head terminals 106 can be monitored (similar to the open-head detection method described above), and then a fault condition triggered when this average voltage exceeds some predetermined level. In the second method, the dramatic increase in IWC that occurs when one of the head terminals 106 is shorted to a power supply or ground can be detected, and used to signal the disk drive system of the short-circuit condition.
An exemplary short-circuit detection (SC detection) circuit 110 (circuitry to the right of the dashed line) for detecting this dramatic increase in IWC in a conventional current-mode write driver under short-circuit conditions is shown in FIG. 1. The SC detection circuit 110 comprises a bias resistor (R1), a low pass filter (R1,C1), an active current source (Q5, R3), a one-shot circuit (INV1, INV2, R5, C2), other bias circuitry (D3-D5, R4), and a D-latch.
Under normal operating conditions, the value of the bias resistor R1 is selected such that the voltage at node WS (IWC*R1) establishes a current in transistor Q5 sufficient to turn on diodes D3-D5. Diodes D3-D5 establish a voltage at the data input of the D-latch that is just below the threshold level of the latch. The invertors INV1, INV2 establish a logic xe2x80x9c0xe2x80x9d at the clock input of the D-latch. Nominal current flows through R4 under normal conditions. The low-pass filter (R2,C1) filters transient noise spikes to prevent the SC detection circuit 110 from registering false short-circuit conditions. The value at the output Whsgf of the D-latch is a logic xe2x80x9c0xe2x80x9d.
When a short-circuit occurs in one (or both) of the write head terminals 106, the magnitude of the short-circuit current IWS exceeds the normal write current IWC supplied by the constant current sources 108 by a significant amount. This added current, which flows through the bias resistor R1, will cause the voltage at the node WS to decrease, thus increasing the current flowing through transistor Q5. The added current flows through R4 and raises the voltage at the data input of the D-latch above the logic threshold level. This, in turn, causes the voltage to rise at the clock input of the D-latch to produce the one-shot signal. The rise time of the clock signal is controlled by the values of R5 and C2. When the voltage at the clock input rises above the logic threshold value, the output Whsgf of the D-latch switches to a logic xe2x80x9c1xe2x80x9d, indicating a short-circuit condition has occurred.
To achieve higher disk drive data storage rates, it is important that the differential output impedance of the write driver match the impedance of the inductive coil assembly. The impedance of the inductive coil assembly is relatively low as compared to the high output impedance associated with current-mode write drivers. This impedance mismatch may cause undesirable undershoot and ringing in the write current transient response, which in turn may cause poor write driver performance and lower data storage rates.
A lower write driver output impedance that better matches the impedance of the inductive coil assembly may be achieved using voltage-mode write drivers. Voltage-mode write drivers establish an IWC in the inductive coil assembly of the inductive head using programmable peak voltage levels, instead of the programmable peak current levels established by current-mode writers. The added complexity of these voltage-mode write driver designs requires that new methods of detecting open and short circuit head terminal conditions be developed.
Accordingly, the present invention addresses a need for providing methods and apparatus to detect terminal open circuits and short circuits to ground in inductive head write drivers.
According to one aspect, an arrangement is provided for detecting a short-circuit condition at at least one of a pair of write head terminals of a write driver, the write driver producing a write current that, when passed through a inductive head assembly coupled to the pair of write head terminals, polarizes the inductive head according to a direction of the write current. The arrangement includes a first current mirror that produces a first current that is proportional to at least a portion of the write current that flows in a first direction into a first write head terminal of the write driver. A second current mirror is included that produces a second current that is proportional to at least a portion of the write current that flows in a second direction, opposite the first direction, into a second write head terminal of the write driver. A short-circuit detection device responsive to the first and second currents is also included. The short-circuit detection device detects a short-circuit condition at at least one of the first and second write head terminals when an average value of the first current is different from an average value of the second current by a predetermined amount.
According to a related aspect, the first and second current mirrors each include two or more individual current mirrors which together mirror the respective at least portions of the write current two or more times in producing the respective first and second currents.
According to another related aspect, a first capacitor is included having a first terminal coupled to an output of the first current mirror and a first input terminal of the short-circuit detection device, the first capacitor having a second terminal coupled to an AC ground terminal. A second capacitor is included having a first terminal coupled to an output of the second current mirror and a second input terminal of the short-circuit detection device, the first capacitor having a second terminal coupled to an AC ground terminal. The first and second capacitors convert the respective first and second currents into corresponding first and second average voltages that are used by the short-circuit detection device to detect a difference in the average value of the first and second currents.
According to yet another related aspect, a first resistor is provided having a first terminal coupled to the output of the first current mirror, the first terminal of the first capacitor, and the first input terminal of the short-circuit detection device, the first resistor having a second terminal coupled to an AC ground terminal. A second resistor is also included having a first terminal coupled to the output of the second current mirror, the first terminal of the second capacitor, and the second input terminal of the short-circuit detection device, the second resistor having a second terminal coupled to an AC ground terminal. The first and second resistors, together with the first and second capacitors, filter out noise present in the respective first and second currents, and wherein the resistors provide respective bias values for the first and second average voltages used by the short-circuit detection device.
According to yet another related aspect, the short-circuit detection device is a differential comparator having an extended linear region that produces a short-to-ground fault signal at an output terminal of the comparator when the difference of the first and second average voltages presented at the input terminals of the comparator exceed the predetermined threshold.
According to yet another related aspect, the at least portions of the write current flowing into the write head terminals in the first and second directions correspond to respective boost currents that each comprise a portion of the overall write current flowing in a given direction for a part of a period needed to polarize the inductive head.
According to yet another related aspect, the magnitude of each of the boost currents is independent of the magnitude of a remaining portion of the overall write current flowing in a given direction for the period needed to polarize the inductive head.
According to another aspect, an arrangement is provided for detecting an open-circuit condition at at least one of a pair of write head terminals of a write driver, the write driver producing a write current that, when passed through a inductive head assembly coupled to the pair of write head terminals, polarizes the inductive head according to a direction of the write current. The arrangement includes a current mirror that produces a mirrored write current that is proportional to the write current that is passed through the inductive head assembly under normal operating conditions. An open-circuit detection device responsive to the mirrored write current is also included. The open-circuit detection device detects an open-circuit condition at at least one of the first and second write head terminals when the magnitude of the mirrored write current drops below a predetermined value.
According to a related aspect, a resistor is provided having a first terminal coupled to an output of the current mirror and a first input terminal of the open-circuit detection device, the first resistor having a second terminal coupled to a reference voltage. The resistor converts the mirrored write current into a voltage that is used by the open-circuit detection device to detect when the magnitude of the mirrored write current drops below the predetermined value.
According to yet another related aspect, the open-circuit detection device is a comparator having an extended linear region that produces an open-circuit fault signal at an output terminal of the comparator when voltage presented at the first input terminal of the comparator exceeds a reference voltage presented at a second input terminal of the comparator by a predetermined threshold.
According to yet another related aspect, the reference voltage presented at the second input terminal of the comparator is inversely proportional to the write current that is passed through the inductive head assembly under normal operating conditions.
According to another aspect, a voltage-mode write driver is provided, including circuitry to produce a write current that, when passed through a inductive head assembly coupled to the write driver through a pair of write head terminals, polarizes the inductive head according to a direction of the write current.
Circuitry is provided to detect a short-circuit condition at at least one of the write head terminals. This circuitry includes a first current mirror that produces a first current that is proportional to at least a portion of the write current that flows in a first direction into a first write head terminal of the write driver. The circuitry also includes a second current mirror that produces a second current that is proportional to at least a portion of the write current that flows in a second direction, opposite the first direction, into a second write head terminal of the write driver. Also a short-circuit detection device responsive to the first and second currents is provided.
Circuitry is also provided to detect an open-circuit condition at at least one of the write head terminals. This circuitry includes a third current mirror that produces a third current that is proportional to the write current that is passed through the inductive head assembly under normal operating conditions. An open-circuit detection device responsive to the third current is provided. The short-circuit detection device detects a short-circuit condition at at least one of the write head terminals when an average value of the first current is different from an average value of the second current by a first predetermined amount, and the open-circuit detection device detects an open-circuit condition at at least one of the write head terminals when the magnitude of the third current drops below a second predetermined value.
According to another aspect, a method is provided for detecting a short-circuit condition at at least one of a pair of write head terminals of a write driver, the write driver producing a write current that, when passed through a inductive head assembly coupled to the pair of write head terminals, polarizes the inductive head according to a direction of the write current. The method includes the step of generating a first current that is proportional to at least a portion of the write current that flows in a first direction into a first write head terminal of the write driver. A second current is generated that is proportional to at least a portion of the write current that flows in a second direction, opposite the first direction, into a second write head terminal of the write driver. A short-circuit condition at at least one of the first and second write head terminals is detected when an average value of the first current is different from an average value of the second current by a predetermined amount.
According to another aspect a method is provided for detecting an open-circuit condition at at least one of a pair of write head terminals of a write driver, the write driver producing a write current that, when passed through a inductive head assembly coupled to the pair of write head terminals, polarizes the inductive head according to a direction of the write current. The method includes the step of generating a mirrored write current that is proportional to the write current that is passed through the inductive head assembly under normal operating conditions. An open-circuit condition at at least one of the first and second write head terminals is detected when the magnitude of the mirrored write current drops below a predetermined value.